/*
 * 图片延迟加载
 * Add on 2012-02-16 by @JefferXia
 * modify on 2012-05-29
 */
(function($) {    
    $.fn.lazyload = function(options) {
        var settings = {
            threshold       : 0,
            failure_limit   : 0,
            event           : "scroll",
            effect          : "show",
            container       : window,
            data_attribute  : "original",
            skip_invisible  : true,
            appear          : null,
            load            : null
        };
                
        if(options) {
            if (undefined !== options.failurelimit) {
                options.failure_limit = options.failurelimit; 
                delete options.failurelimit;
            }
            if (undefined !== options.effectspeed) {
                options.effect_speed = options.effectspeed; 
                delete options.effectspeed;
            }
            
            $.extend(settings, options);
        }

        var elements = this;
        if (0 == settings.event.indexOf("scroll")) {
            $(settings.container).bind(settings.event, function(event) {
                var counter = 0;
                elements.each(function() {
                    $this = $(this);
                    if (settings.skip_invisible && !$this.is(":visible")) return;
                    if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) {
                            
                    } else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) {
                            $this.trigger("appear");
                    } else {
                        if (++counter > settings.failure_limit) {
                            return false;
                        }
                    }
                });
            });
        }
                
        this.each(function() {
            var self = this;
            var $self = $(self);
            
            self.loaded = false;
            
            /* When appear is triggered load original image. */
            $self.one("appear", function() {
                if (!this.loaded) {
                    if (settings.appear) {
                        var elements_left = elements.length;
                        settings.appear.call(self, elements_left, settings);
                    }
                    $("<img />").bind("load", function() {
                            $self.hide().attr("src", $self.data(settings.data_attribute))[settings.effect](settings.effect_speed);
                            self.loaded = true;
                            
                            var temp = $.grep(elements, function(element) {
                                return !element.loaded;
                            });
                            elements = $(temp);
                            
                            if (settings.load) {
                                var elements_left = elements.length;
                                settings.load.call(self, elements_left, settings);
                            }
                      }).attr("src", $self.data(settings.data_attribute));
                };                
            });

            if (0 != settings.event.indexOf("scroll")) {
                $self.bind(settings.event, function(event) {
                    if (!self.loaded) {
                        $self.trigger("appear");
                    }
                });
            }
        });
        
        $(window).bind("resize", function(event) {
            $(settings.container).trigger(settings.event);
        });
        
        $(settings.container).trigger(settings.event);
        
        return this;

    };


    $.belowthefold = function(element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).height() + $(window).scrollTop();
        } else {
            var fold = $(settings.container).offset().top + $(settings.container).height();
        }
        return fold <= $(element).offset().top - settings.threshold;
    };
    
    $.rightoffold = function(element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).width() + $(window).scrollLeft();
        } else {
            var fold = $(settings.container).offset().left + $(settings.container).width();
        }
        return fold <= $(element).offset().left - settings.threshold;
    };
        
    $.abovethetop = function(element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).scrollTop();
        } else {
            var fold = $(settings.container).offset().top;
        }
        return fold >= $(element).offset().top + settings.threshold  + $(element).height();
    };
    
    $.leftofbegin = function(element, settings) {
        if (settings.container === undefined || settings.container === window) {
            var fold = $(window).scrollLeft();
        } else {
            var fold = $(settings.container).offset().left;
        }
        return fold >= $(element).offset().left + settings.threshold + $(element).width();
    };

    $.inviewport = function(element, settings) {
         return !$.rightofscreen(element, settings) && !$.leftofscreen(element, settings) && 
                !$.belowthefold(element, settings) && !$.abovethetop(element, settings);
     };

    /* Custom selectors for your convenience.   */
    /* Use as $("img:below-the-fold").something() */

    $.extend($.expr[':'], {
        "below-the-fold" : function(a) { return $.belowthefold(a, {threshold : 0, container: window}) },
        "above-the-top"  : function(a) { return !$.belowthefold(a, {threshold : 0, container: window}) },
        "right-of-screen": function(a) { return $.rightoffold(a, {threshold : 0, container: window}) },
        "left-of-screen" : function(a) { return !$.rightoffold(a, {threshold : 0, container: window}) },
        "in-viewport"    : function(a) { return !$.inviewport(a, {threshold : 0, container: window}) },
        /* Maintain BC for couple of versions. */
        "above-the-fold" : function(a) { return !$.belowthefold(a, {threshold : 0, container: window}) },
        "right-of-fold"  : function(a) { return $.rightoffold(a, {threshold : 0, container: window}) },
        "left-of-fold"   : function(a) { return !$.rightoffold(a, {threshold : 0, container: window}) }
    });
    
})(jQuery);